Presently pending U.S. patent applications Ser. No. 433,742 filed on Nov. 9, 1989 and the continuation in part thereof, having Ser. No. PCT US 90 06550, filed on Nov. 2, 1990, both being incorporated herein by reference, disclose a new spray conversion process for producing novel nanophase WC-Co composite powders. The process involves three main steps: 1) preparation of starting solutions of mixed salts by wet chemistry methods, 2) spray drying, calcining or roasting of the starting solutions to form homogeneous precursor powders, and 3) thermochemical conversion of the precursor powders to the desired end product powders by controlled gas/solid reactions in a fluid bed reactor. Subsequent consolidation of the powders into useable structural forms may be accomplished by thermal spraying, laser surfacing, cold compaction and sintering, and incipient melt forming.
The thermochemical conversion of the precursor powders according to the process disclosed in these above mentioned patent applications occurs over a period of several hours.
The process disclosed herein, which we refer to as the "carbothermic reaction process," is a modification of the spray conversion process, which permits better control of the WC-Co microstructure at the submicron level and greatly improves conversion efficiency.
The original thermochemical process provided a means for producing nanophase WC-Co composite powders with a composition of 23% by weight cobalt. The steps are outlined below:
1. An aqueous solution of CoCl.sub.2 is mixed with a solution of H.sub.2 WO.sub.4 in ethylenediamine (en) to precipitate crystals of Co(en).sub.3 WO.sub.4, the prototype precursor compound. PA1 2. The crystalline powders are reductively decomposed to form nanoporous/nanophase W-Co powder (see FIG. 1). PA1 3. The high surface area reactive intermediate, W-Co, is converted to WC-Co or other phases by reaction with CO.sub.2 /CO gas mixtures (see FIG. 2). PA1 1. Preparation and mixing of the starting solution. This may take the form of premixing or in situ mixing at the spray drying nozzle. The latter is favored when chemical reaction between the components can occur. PA1 2. Spray drying, calcining or roasting of the starting solution to form homogeneous spherical precursor particles. These may be amorphous, microcrystalline, or mixed amorphous/microcrystalline in nature. PA1 3. Thermochemical conversion of the precursor particles by controlled gas-solid reaction in a fluid bed reactor. This involves control of reaction time, bed temperature, and gas composition. See FIG. 4.
The nature of the microstructure of the composite is determined by controlling the temperature of the carburization reaction and the carbon activity of the gas phase. The resulting powder particles have roughly the same size (10.times.100 microns) and morphology (hexagonal prismatic rods) as the original particles precipitated from solution, but within these particles the microstructure is a WC-Co nanophase composite, FIG. 3.
Using Co(en).sub.3 WO.sub.4 as the precursor compound necessarily fixes the Co/W atom ratio at 50/50 and the resulting WC-Co composition at 23 weight percent Co. This composition is at the low end of WC loadings that are used commercially. Thus, there is a need to extend the compositional range of precursors to include more tungsten. The range of WC-Co compositions of commercial interest is 3-30 weight percent Co.
To overcome this limitation in the original thermochemical process, we have adopted spray drying or solution mixtures as the preferred method of making precursor powders with a range of compositions. In spray drying the solvent phase is rapidly evaporated in a hot gas stream, leaving solid particles that are homogeneous mixtures. Under ideal conditions the solid particles are amorphous or microcrystalline, with no evidence of phase separation, even when starting from multicomponent solutions.
To summarize, our "Spray Conversion Processing" technology involves:
In preparing the precursor powders the preferred starting point is an aqueous solution of ammonium metatungstate (AMT), (NH.sub.4).sub.6 (H.sub.2 W.sub.12 O.sub.40).4H.sub.2 O and cobalt nitrate, Co(NO.sub.3).sub.2.6H.sub.2 O. AMT was chosen because among the polytungstates, it has the highest solubility in water. Water soluble Co(NO.sub.3).sub.2.6H.sub.2 O was selected because it decomposes to form non-corrosive NO.sub.x compounds, which are easily scrubbed from the system. Chloride compounds, if used, can cause corrosion of the metal components of the reactor.
The Co/W atom ratio was adjusted to 1.0, 0.63, 0.21, and 0.1 by mixing appropriate quantities of AMT and cobalt nitrate. Spray drying and thermochemical conversion in CO.sub.2 /CO gas at a carbon activity of 0.95 yields the resulting nanophase WC-Co powders having 23, 12, 6, and 3 wt % Co binder phase, respectively. The particle microstructure of these powders was substantially the same as that obtained for WC-Co composite made from Co(en).sub.3 WO.sub.4 powder.
The thermochemical conversion of precursor powders in a fluid bed reactor has been a substantial improvement in the technology. While one can obtain sufficient powder for characterization purposes using a laboratory-scale fixed bed reactor, it is not easy to obtain the larger quantities needed for mechanical property evaluations, much less produce commercial quantities of powders for field testing. The difficulty has been circumvented by adopting an industrial-scale fluid bed reactor as the means for controlled thermochemical conversion of the precursor powder to WC-Co nanophase composite powder. A fluid bed reactor is ideal for thermochemical conversion of the precursor powder because of the uniform bed temperature and constant gas/solid environment throughout the bed.